855 



CHAPTER XY 



SPONGES AND ZOOPHYTES 



I. SPOXGES 



WE now leave the PROTOZOA and commence the study of the METAZOA, 

 or those forms in which the egg-cell undergoes subdivision, the result- 

 ing elements of which do not separate or lead an independent 

 existence, but combine to form an organic whole, various parts 

 undertaking various functions. Of these Metazoa the simplest ex- 

 amples are to be found among SPONGES. The determination of the 

 real character of the animals of this class has been entirely effected by 

 the microscopic examination of their minute structure ; for until this 

 came to be properly understood, not only was the general nature of 

 these organisms entirely misapprehended, but they were regarded 

 by many naturalists as having no certain claim to a place in the 

 animal kingdom. What that place is, is, to some extent, still an 

 open question, 1 but it may now be unhesitatingly affirmed that a 

 sponge is an aggregate of protozoic units only in the sense in which 

 ,ill -Met; i /on are composed of cells ; some of these cells have a striking 

 resemblance to the collared Flayellata (fig. 585), whilst others re- 

 semble Amcebce (fig. 577). These units are held together by a con- 

 tinuous connective-tissue-like substance which clothes the skeletal 

 framework that represents our usual idea of a sponge, and is itself 

 made up of distinct cellular elements. In the simpler forms of 

 sponges, however, this frame\vork is altogether absent ; in others it 

 is represented only by calcareous or silicious ' spicules,' which are 

 dispersed through the sarcodic substance (fig. 654, B) ; in others, 

 again, the skeleton is a keratose (horny) network, which may be 

 entirely destitute (as in our ordinary sponge) of any mineral support, 

 but which is often strengthened by calcareous or silicious spicules 

 (fig. 654) ; whilst in what may be regarded as the highest types of 

 the group, the silicious component of the skeleton increases, and the 

 keratose diminishes until the skeleton consists of a beautiful silicious 

 network resembling spun glass. But whatever may be the condi- 

 tion of the skeleton, that of the body that clothes it remains 



1 For an instructive discussion on this point, consult Prof. E. A. Minchin's essay 

 on ' The Position of Sponges in the Animal Kingdom' in Science Papers, i. (n.s.) 

 (1897), to which is appended a useful list of works on the subject. Some axithors 

 demur to the association of sponges with other Metazoa, and Professor Sollas has sug- 

 gested the use of the group-name Parazoa. See also Treatise on Zoology, vol. ii. 

 London, 1900. 



